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phosphoric acid

phosphoric acid, any one of three chemical compounds made up of phosphorus, oxygen, and hydrogen (see acids and bases). The most common, orthophosphoric acid, H3PO4, is usually simply called phosphoric acid. Two molecules of it are formed by adding three molecules of water, H2O, to one molecule of phosphorus pentoxide (phosphoric anhydride, P2O5). It occurs as rhombic crystals or as a viscous liquid; both are deliquescent. The crystals melt at about 42°C. It has specific gravity 1.834 at 18°C, is soluble in alcohol, and is very soluble in water. It is a tribasic acid and forms orthophosphate salts with either one, two, or all three of the hydrogens replaced by some other positive ion. When it is heated to about 225°C, it dehydrates to form pyrophosphoric acid, H4P2O7; at still higher temperatures metaphosphoric acid, HPO3, is formed. Salts of pyrophosphoric acid are pyrophosphates; salts of metaphosphoric acid are metaphosphates. Phosphoric acid is prepared commercially by heating calcium phosphate rock with sulfuric acid; purer grades may be prepared by treating red phosphorus with nitric acid. It is used in pickling and rust-proofing metals, in acidifying jellies and beverages, and in preparing phosphate salts.

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phosphoric acid

phosphoric acid Group of acids. Tetraoxophosphoric acid (H3PO4, formerly orthophosphoric acid) is a colourless liquid obtained by the action of sulphuric acid on phosphate rock (calcium phosphate); it is used in dental adhesives, flavoured syrups, fertilizers, soaps, detergents, and anticorrosive coatings for metals. Metaphosphoric acid (HPO3) is obtained by heating tetraoxophosphoric acid; it is used as a dehydrating agent. Heptaoxodiphosphoric (H4P2O7, formerly pyrophosphoric acid) is formed by moderately heating tetraoxophosphoric acid or by reacting phosphorus pentoxide (P2O5) with water; it is used as a catalyst and in metallurgy.

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phosphoric acid

phosphoric acid May be one of three types: orthophosphoric acid (H3PO4), metaphosphoric acid (HPO3), or pyrophosphoric acid (H4P2O7). Orthophosphoric acid and its salts are used as acidity regulators and in acid‐fruit‐flavoured beverages.

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